Feedback system for reed modulated magnetrons



April 23, 1963 w. w. MCLEOD, JR 3,0875124 FEEDBACK SYSTEM FOR REEDMODULATED MAGNETRONS Filed May 29, 195e M//u/Wo W Mdsoa, J2

,4 rroe/vgy 3,087,124 Patented Apr. 23, 1963 ice 3,087,124 FEEDBACKSYSTEM FOR REED MODULATED MAGNETRQNS Willard W. McLeod, fr., Lexington,Mass., assigner to Raytheon Company, Lexington, Mass., a corporation ofDelaware Filed May 29, 1958, Ser. No. 738,892 Qlaizns. (Cl. 332-16) Thisinvention relates to electron ldischarge devices, and more particularly,to feedback means for stabilizing the frequency modulationcharacteristics of tunable election discharge devices of the frequencymodulated megnetron type.

In an electron discharge device of the vibratory reed megnetron type, inwhich the frequency of the megnetron is a function of the capacityIbetween the anode straps, the vibratory reed or tunable element usuallysupports a sensing coil and .a drive coil adjacent to the anode and ismovable with respect to the anode straps for tuning the device. As isknown, the sensing or pick-up coil senses the motion of the vibratoryreed or tunable element and produces a feedback voltage which iscombined with a modulating input signal and amplified in `a feedbackamplifier. The output of this amplifier is connected to the drive coilto produce an electromagnetic field which interacts with the magneticfield of the magnetron to impart motion to the reed. In this manner, thedrive coil operates to damp and control the motion of the reed toconform more accurately to the input frequency modulating signal.However, since the magnetron frequency is a function of the spacingbetween the reed or tunable element and the anode, a two coil system ofthis type does not directly compensate for variations in Spacing betweenthe reed and the anode. As a result, the feedback voltage so developedcontrols only the motion of the reed and not the capacity between thereed and the anode straps.

In addition, undesirable transformer coupling between the drive and thepick-up coils tends to cancel out the feedback voltage and limit thefrequency response of the feedback system so that it is difficult, ifnot inrpossible, to control the motion of the reed at high frequencies.As a result, the frequency response of the tunable element or reedmodulator is Seriously limited in the region in which the feedbackvoltage is impaired. It is therefore desirable to provide a feedbacksystem in which the capacity between the anode straps of a frequencymodulated magnetron is varied in a linear manner in order to improve thefrequency response of the reed modulator and, thereby, obtain linearfrequency modulation.

In accordance with the reed modulated feedback system of this invention,a capacity plate is insulatedly supported by the vibratory reed ortunable element and positioned adjacent to the straps of a frequencymodulated magnetron in order to form a capacity pick-off to develop afeedback voltage. This feedback voltage is fed to the input stage of afeedback amplier connected to a drive coil supported by the vibratoryreed element. The capacity plate is insulated from and biased withrespect to the anode of the magnetron by means of a high impedance inputcircuit which introduces the frequency modulated input signal and thefeedback voltage into the feedback amplier. The time constant of thisinput circuit is made short relative to the input modulating frequencyto permit the feedback voltage signal to be proportional -to the rate ofchange of capacitance between the capacity plate and the anode straps.This feedback then provides frequency modulation Which is proportionaland linear with the input modulating signal. In addition, the voltagederived by the capacity pick-olf plate Will be a function of one overthe reed to anode spacing, which is the same as the frequency modulatingcharacteristics of the vibratory reed element. In this manner, theeffective capacity between the straps of the magnetron is controlled Abya capacity feedback system of this type and, thus, compensation isintroduced for the non-linearity of the reed modulation element. Withthis arrangement, the rate of change of frequency of the magnetron isproportional to and linear with the input signal to the amplifier. Inaddition, the direct coupling between the drive and pick-up elements andthe resulting limitation on frequency response is substantiallyeliminated.

The system of this invention constitutes an improvement over the systemdisclosed in U.S. Patent No. 2,750,565 of William R. Mercer et al.,issued June 12, 1956.

Other and further advantages of this invention will be apparent as thedescription thereof progresses, reference being had to the accompanyingdrawings, wherein:

FIG. l is la fragmentary longitudinal sectional view taken through thecenter of the magnetron of the vibrating reed type showing the capacityplate and driving coil on the tunable reed element;

FIG. 2 is a simplified schematic of the magnetron feedback circuit ofthe subject invention in which the capacity feedback plate is connectedin a manner to affect the input signal to the driving coil, hereinafterreferred to as a closed loop circuit; and

FIG 3 is an enlarged, fragmentary View of a portion of FiG. 1 showingthe vibratory reed element and the capacity plate.

FIG. 1 illustrates a frequency modulated magnetron, generally indicatedby reference numeral 10, which utilizes a vibrating reed assembly uponwhich a driving coil and a capacity plate are mounted. Magnetron 10comprises an anode structure 11, a cathode structure 12 (only partiallyshown), magnetic means including pole pieces 13 and 14 for establishinga magnetic eld in a direction perpendicular to the path of theI electronflow between said cathode and anode structures, and a vibratory reedassembly or tunable element 15.

As shown in FIG. 1, anode structure 11 includes a cylindrical body 16made of highly conductive material, such as copper, and having aplurality of radially extending anode vanes 17, as is Well known in themagnetron art. Surrounding `the cylindrical body 16 is a cover assembly18 which functions as a magnetic return path for pole pieces 13 and 14.Anode structure 11 is closed at the ends, for example, by end plates 19and 20 hermetically sealed at the junction between said plates andcylindrical body 16, as shown at 21. The cathode structure 12 iscoaxially arranged with respect to the anode structure and comprises acathode heating filament 22 which passes through a recess in the polepiece 14 in the usual manner. The cathode may be supported and insulated`from the anode structure in the manner shown by Becker in U.S. PatentNo. 2,566,478, dated September 4, 1951.

One of the pole pieces 13 is hermetically sealed, as at 25, into the endplate 19. The other pole piece 14 is broken `away at 26 and shows acentrally located bore which allows for entry of cathode assembly 12.This pole piece may be hermetically sealed as at 27 into the end plate2li. Pole pieces 13 and 14- are adapted to be fixed to the opposite endsof a magnet, such as bar magnets (not shown) for producing anappropriate magnetic field, as noted, in a direction normal to the pathof electrons flowing from cathode to anode.

A coupling device 28 connected to one of the anode vanes `17 serves tocouple energy out of the magnetron and 3 is brought out through outletpipe 30 `which engages cylindrical body `lla of anode structure il andextends through the cover assembly 18.

The anode -vanes 17 are each provided With a slot 31 f or receiving `apair'of concentric conducting straps 32 and 33 which contact alternateanode vanes. As is well known in the magnetron art, the aforesaid strapspresent a capacitance therebetween which partially determines thenatural resonant frequency of the magnetron.

Referring now to FIGS. l and 3, a vibrating reed assembly 15 includes acapacity plate 34 which is moved with respect to anode vanes 17, that iswith respect to straps 32 and 33, whereby the distributed capacitancemay be altered so that the magnetron may be Ivaried in frequency. vThecapacitance or capacity plate 34 takes the form of a metalized coatingon an annular ceramic diaphragm 40 which is secured to the lower flangedportion of a hollow cylindrical core 36. The core 36 forming part of thetunable reed element 15 is secured, as by soldering, to a ring or fiangeportion 37 of the tunable element which, in turn, is secured at itsperiphery, to a supporting ledge 38 of Ycylindrical body d6 of anodestructure 1l.

The capacitive plate 34, as shown in FIG. 3, consists of a metalizedsurface on the ceramic insert ttl secured to reed-like flange 46partially surrounded by an extension of corre 36 and adjacent to corebracket 39. The end of core 36 opposite from core bracket 39, is bracedby means of support 4S extending diagonally to the vibrator portion 49of the reed-like ange 46. The ceramic support 40 insulates thecapacitive plate 34 from the tuning assembly 15 and, in particular, fromthe core or metallic support 3bl for a drive coil 5t). Connected to themetallic capacitive plate is =a wire output lead 41 which passes throughan opening 4Z in anode ring or fiange 37 and out yof the magnetronthrough la glass seal i3 in the end plate 19. The output lead il thenpasses through an opening 4 4 in cover assembly 18 and, together with alead i5 connected to the anode assembly, provides a means forintroducing the feedback voltage into the input circuitry of thefeedback amplifier.

The drive coil 50, `as shown in FiG. 3, preferably consists ofapproximate-1y 20y turns of insulated wire which is wound upon thehollow core 36 and firmly attached thereto. The ends of drive coil 50are led out of the magnetron through an aperture in the ring or flangeportion 37 of the tunable element and through a glass seal 52 fused intothe upper end plate i9. Output leads 54 and 55 are connected to the endsof coil 5t? extending beyond the glass seal 52. The tunable reedassembly i5 and the driving coil Sil carried lthereon are disposedrelative to the pole pieces 13 and 14 so that the drive coil 5G is inthe leakage field existing between said pole pieces. When an externalmodulating voltage is applied to leads 54 and 55, the electromagneticfield set up between the current flowing in driving coil 5f) and theleakage magnetic field of the magnetron interact to produce motion ofcore 36 and is accompanying capacitive plate 34 with respect to theanode straps 32 and 33. in this manner, the distributed capacitanceofthe magnetron is varied and frequency modulation obtained. As thetuning element vibrates to and fro in response to the drive coil input,a capacitive pick-off voltage is generated on the capacitive plate 34which is fed by way of lead di to the input terminals of a feedbackamplifier 6l.

Referring to FIG. 2, a square wave applied to the input terminals 6'9,60 is coupled to feedback amplifier 6i by way of an input circuit 62.This circuit comprises a .5 megohm input resistor 63, and a .lmicrofared coupling capacitor 54 which isolates the negative biasvoltage from the input of amplifier 61. The insulated capacitive plate34g isl biased at a negative voltage of minus 400 vo'rts with respect-to the grounded anode structure 17 by means of a bias source 65 and .5megohm bias resistor 66. The out put of amplifier all is coupled throughan output transformer 68` to the driving coil Si? shown in HG. l. Thecore about which the drive coil 50 is securely wound is adapted to moveto and fro between the magnetic pole pieces )i3 and 14;- as shown by thearrows. The feedback voltage which is developed by the capacity plate 34is fed to the input circuit of the amplifier, the time constant of whichis made short relative to the square wave input modulating lvoltage atterminals 6i), 60. In this closed loop circuit, the feedback voltage andinput signal become proportional to the rate of change of capacitancebetween the capacity plate 34 and the anode straps 32 and 33. Thisfeedback circuit, therefore, provides a rate of change of frequencywhich is proportional to the input modulating signal. At the same time,the capacity plate provides that the feedback voltage is a function ofone over the reed to anode spacing which is the same as the modulationcharacteristics of the vibrating tunable e1e ment. Thus, any variationof the reed modulation ele ment due to non-linearity is compensated forby the arnplified feedback and signal voltages. The voltage out put fromthe capacitor plate will thus be constant while the motion of the coreis in one direction, changes polarity when the coil reverses direction,and remains constant at the new polarity until the cycle reoccurs. Iffor any reason, Ithe motion of the drive coil is not linear, the erroror feedback voltage derived across the capacitive plate is fed to theamplifier input circuit7 combined with the input signal voltage andreamplified in phase opposition so as to tend to cancel out the errororiginally generated by irregularities in the drive coil motion.

It should be understood that this invention is not limited to theparticular details of construction, materials and processes described,as many equivale-nts will suggest themselves to those skilled in theart. For example, the electrically insulated capacity plate may be usedto regulate the frequency response of other than electron dischargedevice-s. The non-linear response of, for example, a driving coilapplied to a magnetic transducer, in an arrangement as shown in FG. 3,may be compensated by the feedback voltage developed in this closed loopfeedback system, or other non-linear electromechanical vibratingassemblies may attain similar advantages.

It is accordingly desired that this invention not be limited to theparticular details of the embodiment disclosed herein except as definedby the appended claims.

What is claimed is:

1. A frequency modulation circuit comprising an electron dischargedevice including a cathode, an anode, means for producing a magneticfield and a tunable element positioned in said magnetic field, saidtunable element including a conductive plate insulatedly supportedthereon and 'adjacent to said anode in a manner adapted to vary thefrequency of said electron discharge device in response to movement ofsaid plate, means for varying the position of said plate including adriving element mounted on said tunable element, means for applying aninput voltage to said driving coil, means for developing a feedbackvoltage in response to variations in electrical charge on said plate dueto changes of capacitance between said plate and said anode, and meansfor combining said feedback voltage with said input Voltage to apply amodulating voltage to said driving element.

2. A frequency modulation circuit comprising means for establishing aflow of electrons in a given direction, means for establishing amagnetic field in a direction substantially perpendicular to suchdirection of electron flow, and means mounted for movement within suchmagnetic field and adjacent such electron fiow to both frequencymodulatesuch electron flow in accordance with such movement and provide acapacitive pick-off the amplitude of which varies in linear relationshipwith such movement, whereby the variable capacitance of said pick-offmay be employed to control feedback circuitry the output of which iscombined with drive signals applied to said frequency modulating meansto stabilize the operation of such lastmentioned means.

3. A frequency modulation circuit comprising means for establishing aiiow of electrons in a given direction, means for establishing amagnetic field in a direction substantially perpendicular to suchdirection of electron iiow, means mounted for movement within suchmagnetic field and adjacent such electron flow to bothfrequency-modulate such electron flow in accordance with such movementand provide a capacitive pick-olf the amplitude of which varies inlinear relationship with such movement, means for effecting movement ofsaid frequency modulating means in accordance with a modulating signal,and means connected in feedback relationship between said capacitivepick-off means and said means for effecting movement.

4. A frequency modulation circuit, comprising an electron dischargedevice including a cathode, an anode, and means for establishing amagnetic field in a direction substantially perpendicular to the path ofthe electron flow between said cathode and said anode, a mechanicaltuning means positioned in said magnetic iield, and a conductive plateinsulatedly supported -by said tuning means within said device incapacitive relationship with said anode, said tuning means being mountedfor movement relative to said anode to vary the distance between saidconductive plate and said anode and thereby vary such capacitive`relationship linearly with respect to such movement, whereby themovement of said conductive plate serves both to frequency-modulate theelectron ow from said cathode to said anode and to provide -a capacitivepickoff which varies linearly with such movement.

5. A frequency modulation circuit comprising an electron dischargedevice having a cathode, an anode yand a magnetic circuit having a fieldportion substantially perpendicular to a ow of electrons between saidcathode and said anode, a vibratory member mounted for movement withinsuch magnetic eld adjacent such electron flow, a drive coil mounted onsaid vibratory member, an input circuit for said drive coil for applyingmodulating signals thereto to cause corresponding movement of said coiland vibratory member as la result of interaction between the magneticfields of said coil and said magnetic circuit, a conductive plate membermounted on and insulated from said vibratory member for movement toward-and away from said anode in capacitive relationship therewith, and afeedback circuit connected between said insulated conductive platemember and said input circuit, whereby such movement of said conductiveplate member serves both to frequency-modulate the electron flow fromsaid cathode to said anode and to provide a variable capacitance controlfor such feedback circuit which varies linearly with the movement ofsaid conductive plate member.

References Cited in the le of this patent UNITED STATES PATENTS2,114,086 Smith et al Apr. 12, 1938 2,269,041 Schussler Jan. 6, 19422,444,435 Fisk July 6, 1948 2,493,819 Harry Jan. 10i, 195o 2,615,156Smith Oct. 2l, 1952 2,750,565 Mercer June 12, 1956 2,784,345 SpencerMar. 5, 1957 2,802,178 Shafer Aug. 6, 1957

1. A FREQUENCY MODULATION CIRCUIT COMPRISING AN ELECTRON DISCHARGEDEVICE INCLUDING A CATHODE, AN ANODE, MEANS FOR PRODUCING A MAGNETICFIELD AND A TUNABLE ELEMENT POSITIONED IN SAID MAGNETIC FIELD, SAIDTUNABLE ELEMENT INCLUDING A CONDUCTIVE PLATE INSULATEDLY SUPPORTEDTHEREON AND ADJACENT TO SAID ANODE IN A MANNER ADAPTED TO VARY THEFREQUENCY OF SAID ELECTRON DISCHARGE DEVICE IN RESPONSE TO MOVEMENT OFSAID PLATE, MEANS FOR VARYING THE POSITION OF SAID PLATE INCLUDING ADRIVING ELEMENT